This invention relates to a method of and apparatus for treating nitrogen oxides in a gas turbine exhaust gas.
In a conbustor for a gas turbine, nitrogens in air introduced into the combustor are oxidized in its high temperature area to form nitrogen oxides (will be described hereinafter simply NO.sub.x). A combustion gas including the NO.sub.x is exhausted into the atmosphere through the turbine. The NO.sub.x are poisonous to living organisms and cause a photochemically generated smog, so that prevention thereof is highly demanded. In case of the gas turbine, as methods of prevention of NO.sub.x generation known are the followings;
(1) method of preventing of NO.sub.x generation, PA1 (2) method of reducing and decomposing the NO.sub.x. PA1 (a) to add non-combustion materials such as water and steam into the combustor, PA1 (b) combustion controls such as lean fuel-air mixture combustion and two step combustion.
The method of 1) now is being put into a practice. The method is effected by lowering a combustion temperature since the NO.sub.x are formed proportionally to a reaction temperature, oxygen and nitrogen densities of the surroundings, and reaction continuity time, particularly the production of the NO.sub.x is exponentially proportional to the reaction temperature. The concrete methods for lowering the combustion temperature are as follows;
According to the method of 1), even if so-called thermal NO.sub.x which are formed by high temperature combustion could be reduced, fuel NO.sub.x which are formed by oxidation of nitrogen compounds contained in a fuel can not be reduced. By the method of lowering combustion temperature by adding water or steam, an amount of HC and CO generated in the combustion increases, therefore fuel consumption rate is raised whereby turbine effeciency is decreased. Further by effecting low temperature combustion employing an excess air, problems such as going out of combustion and unstable combustion occurences are accompanied, therefore it is more difficult to expect decreasing effect of the NO.sub.x than the method of adding water or steam.
Thus, there are various problems in the methods of 1), so that it is difficult to treat the exhaust gas from the turbine only by the method of 1).
On the other hand, as a method of 2) that is a method of reducing and decomposing the NO.sub.x in the exhaust gas, a method in which the NO.sub.x are reduced and decomposed to N.sub.2, CO.sub.2 and H.sub.2 O by reaction of the NO.sub.x with reducing gases such as CO, H.sub.2, NH.sub.3 under an oxidation catalyst is known for example by U.S. Pat. Nos. 2,975,025 and 3,008,796 and Japanese patent publication No. 44-13002. The exhaust gas from the gas turbine includes about 15% of an excess oxygen therefore when the CO and H.sub.2 in the above reducing gases are used for treatment of the exhaust gas, the CO and H.sub.2 react with the oxygen before reduction of NO.sub.x so that a large amount of the CO and H.sub.2 is necessary, which is uneconomical.
In case of the NH.sub.3, however in a certain temperature range, reduction reaction of the NO.sub.x with the NH.sub.3 is effected faster than oxidation of the NO.sub.x, which is called a selective catalytic reduction reaction of the NO.sub.x. Namely, the reaction of reduction of the NO.sub.x with the NH.sub.3 is effected between temperatures of 250.degree. C. and 450.degree. C. in the presence of catalyst. When the reaction temperature is higher than 450.degree. C., the reaction speed of oxidation of the NH.sub.3 is faster and the oxidation of the NH.sub.3 takes place preferentially than the reaction of reduction of the NO.sub.x to result in lowering or denitration efficiency. When the reaction temperature is lower than 250.degree. C., the reaction speed of reduction of the NO.sub.x is lowered to also result in lowering of denitration efficiency.
Thus, since the NH.sub.3 reduces selectively the NO.sub.x it is suitable for treatment of the NO.sub.x in the presence of a large amount of oxygen, but it also has a defect that the temperature range for effective denitration is limited.
Temperature of combustion gas used for the turbine is about 1000.degree. C., and temperature of the exhaust gas exhausted from the turbine is about 500.degree. C. However, now the combustion gas temperature tends to be further elevated, and corresponding to the increasing combustion gas temperature, the exhaust gas temperature also is elevating. Accordingly the temperature of the reaction of reduction of the NO.sub.x should be effectively controlled without decrease of turbine efficiency so that the denitration efficiency will be sufficient.
As the selective catalytic reaction of reduction of the NO.sub.x in the presence of the O.sub.2, urea, hydrazine, etc. are known besides the ammonia. The hydrazine carries out nitrogen oxide reduction efficiency of 70-75%, under 450.degree.-550.degree. C. of reaction temperature, and without catalyzer; the urea has about 80% nitrogen oxide reduction efficiency under 450.degree.-500.degree. C. of the reaction temperature and in the presence of a proper catalyst.